At the 15th annual Nikon conference, Dr. Martin Weiss, Principal Engineer with Intel’s Portland Technology Development group, highlighted the importance of overlay and underlay control in continuing 193 nm immersion scaling. Overlay control is a major focus for advanced process nodes; and he emphasized tight control is needed for optimal yield and that every nanometer counts. Meeting the overlay requirements is increasingly difficult as those requirements scale faster than the layout pitch if multiple patterning is employed. Weiss noted that these multiple patterning factors impact the litho scanner alignment as well as the associated overlay metrology.
Figure 1A. Pattern decomposition with multiple patterning allows a path beyond the standard 193 nm immersion resolution limits (left image). Figure 1B. Complicated align trees lead to more chances for yield issues.
Pattern decomposition with multiple patterning allows a path beyond the standard 193 nm immersion resolution limits (Figure 1A), but it also increases the number of underlayers that the current layer must be controlled to. Weiss explained that this implies overlay requirements that are scaling faster than Moore’s Law. Overlay control is imperative since the increasing number of align-pairs that must be controlled leads to even more chances for yield issues (Figure 1B). Overlay capability at the current layer is affected by the relative alignment of the align-to layers (also known as the underlay). This makes it necessary to reduce the alignment tree complexity in order to recover overlay capability. Weiss noted this can be achieved by processing solutions such as self-aligned double patterning (SADP), which adds process cost; and by coupling SADP with scanner solutions like EUV, which has very high equipment cost (Figure 2A).
Figure 2A. Alignment tree complexity can be reduced by process and scanner solutions (left image). Figure 2B. Litho scanner capabilities play a key role in solving overlay challenges.
Weiss reported that although some overlay problems can be solved by the process and/or the design, the litho system’s capabilities play a key role. The complicated alignment trees and yield sensitivity to overlay necessitate tight scanner overlay control, while the complex film stacks impact the alignment signals on the scanner as well. There has been steady improvement in scanner overlay capabilities, with roughly a 3x reduction in mix-and-match overlay over the last few generation systems (Figure 2B). Also, Weiss noted that Nikon Super Distortion Matching (SDM) provides control for high (3rd and 4th) order overlay terms and typically recovers ~2 nm of overlay performance, but can correct for even more.
In addition, the Nikon inline Alignment Station (iAS) that was introduced on the NSR-S631E immersion scanner provides dense alignment sampling on an integrated metrology station. iAS enables rapid collection of detailed alignment data without gating scanner throughput. It is a multi LED-based system that provides enhanced field image alignment (FIA) with dark field imaging capabilities, as well as a variable numerical aperture (NA), and variable polarization. The extremely dense iAS alignment data is supported in KLA 5DA software with data mining, signal analysis and optimization, and mark selection. Weiss explained that multiple patterning involves intricate film stacks with more films and materials, and stressed that the scanner alignment system needs to provide a variety of illumination options to enable overlay control across that broad range of stack materials (Figure 3A).
Figure 3A. Complex process stacks necessitate robust alignment systems (left image). Figure 3B. The Nikon iAS system enhances performance with active control of wafer-level and field-level overlay.
Historically scanner recipes have been set to measure one alignment mark per field, thus limiting active control of overlay to wafer grid corrections. Field-level overlay was controlled/monitored by a feedback loop from downstream metrology. With the Nikon iAS system, high density alignment measurements can be made without impacting scanner throughput. This enables active control of wafer-level and field-level overlay during alignment to significantly enhance performance (Figure 3B).
Weiss then discussed the role of overlay metrology, which provides the feedback loop to control the scanner using image-based overlay (IBO) and diffraction-based overlay (DBO) techniques. Metrology measurement accuracy is critical, and there are challenging constraints on the measurement structures, i.e. the overlay metrology is required to provide data for specific align-pairs in the alignment tree (Figure 4A). Additionally, while alignment errors from the scanner will be caught by the overlay metrology tool, there is no containment for overlay tool measurement errors, making that area a possible quality risk.
Figure 4A. Overlay metrology presents a number of quality risks (left image). Figure 4B. Metrology error combined with process effects can induce underlay problems.
Ensuring measurement quality is crucial, and there are many measurement options to address complex stacks. Accuracy and robustness are vital focus factors. Although metrology may be possible under many conditions, only a small subset of them are sufficiently robust to processing variations in film thickness, uniformity, profiles, etc., and even fewer are accurate enough. Weiss warned that metrology errors can quickly consume most of the overlay budget, and reiterated that the ability to control overlay depends on the measurement quality. He then showed how metrology error combined with process bias effects can induce underlay problems as well as impact the overlay capability of downstream layers (Figure 4B).
Weiss concluded his informative presentation by underscoring that multiple patterning enables the extension of 193 nm immersion lithography CD/pitch scaling, but places higher demands on overlay control. Lithographers must manage the complexity of the align trees with process and scanner solutions to meet the aggressive overlay requirements. Advanced scanner alignment solutions such as iAS and SDM provide dense data sampling and high order control, while variable illumination alignment accommodates a broad range of substrates. In closing, he reminded the audience that metrology measurement quality and film stack robustness are essential for overlay control as well.